If you are using a fixed navbar, you can use the is-fullheight-with-navbar modifier on the hero for it to occupy the viewport height minus the navbar height.
<sectionclass="hero is-primary is-medium"><!-- Hero head: will stick at the top --><divclass="hero-head"><navclass="navbar"><divclass="container"><divclass="navbar-brand"><aclass="navbar-item"><imgsrc="https://bulma.io/images/bulma-type-white.png"alt="Logo"></a><spanclass="navbar-burger"data-target="navbarMenuHeroA"><span></span><span></span><span></span></span></div><divid="navbarMenuHeroA"class="navbar-menu"><divclass="navbar-end"><aclass="navbar-item is-active">
Home
</a><aclass="navbar-item">
Examples
</a><aclass="navbar-item">
Documentation
</a><spanclass="navbar-item"><aclass="button is-primary is-inverted"><spanclass="icon"><iclass="fab fa-github"></i></span><span>Download</span></a></span></div></div></div></nav></div><!-- Hero content: will be in the middle --><divclass="hero-body"><divclass="container has-text-centered"><pclass="title">
Title
</p><pclass="subtitle">
Subtitle
</p></div></div><!-- Hero footer: will stick at the bottom --><divclass="hero-foot"><navclass="tabs"><divclass="container"><ul><liclass="is-active"><a>Overview</a></li><li><a>Modifiers</a></li><li><a>Grid</a></li><li><a>Elements</a></li><li><a>Components</a></li><li><a>Layout</a></li></ul></div></nav></div></section>
<sectionclass="hero is-success is-fullheight"><!-- Hero head: will stick at the top --><divclass="hero-head"><headerclass="navbar"><divclass="container"><divclass="navbar-brand"><aclass="navbar-item"><imgsrc="https://bulma.io/images/bulma-type-white.png"alt="Logo"></a><spanclass="navbar-burger"data-target="navbarMenuHeroC"><span></span><span></span><span></span></span></div><divid="navbarMenuHeroC"class="navbar-menu"><divclass="navbar-end"><aclass="navbar-item is-active">
Home
</a><aclass="navbar-item">
Examples
</a><aclass="navbar-item">
Documentation
</a><spanclass="navbar-item"><aclass="button is-success is-inverted"><spanclass="icon"><iclass="fab fa-github"></i></span><span>Download</span></a></span></div></div></div></header></div><!-- Hero content: will be in the middle --><divclass="hero-body"><divclass="container has-text-centered"><pclass="title">
Title
</p><pclass="subtitle">
Subtitle
</p></div></div><!-- Hero footer: will stick at the bottom --><divclass="hero-foot"><navclass="tabs is-boxed is-fullwidth"><divclass="container"><ul><liclass="is-active"><a>Overview</a></li><li><a>Modifiers</a></li><li><a>Grid</a></li><li><a>Elements</a></li><li><a>Components</a></li><li><a>Layout</a></li></ul></div></nav></div></section>